3GPP LTE radio interface offers high peak data rates, low delays, and an increase in spectral efficiencies. The LTE ecosystem supports both frequency division duplex (FDD) and time division duplex (TDD), allowing the operators to exploit both the paired and unpaired spectrum, since LTE has flexibility in bandwidth as it supports 6 bandwidths: 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz.
The LTE physical layer is designed to achieve higher data rates, and is facilitated by turbo coding/decoding and higher order modulations (up to 64-QAM). The modulation and coding is adaptive, and depends on channel conditions. Orthogonal frequency division multiple access (OFDMA) is used for the downlink, while single carrier frequency division multiple access (SC-FDMA) is used for the uplink. The main advantage of such schemes is that the channel response is flat over a sub-carrier even though the multi-path environment could be frequency selective over the entire bandwidth. This reduces the complexity involved in equalization, as simple single tap frequency domain equalizers can be used at the receiver. OFDMA allows LTE to achieve higher data rates, reduced latency, and improved capacity/coverage with reduced costs to the operator. The LTE physical layer supports HARQ, power weighting of physical resources, uplink power control, and MIMO. By using multiple parallel data streams transmission to a single terminal, data rates can be increased significantly.
In a multi-path environment, such a multiple access scheme also provides opportunities for performance enhancing scheduling strategies. Frequency Selective Scheduling (FSS) can now be used to schedule a user over sub-carriers (or part of the bandwidth) that provides maximum channel gains to that user (and avoid regions of low channel gain). The channel response is measured and the scheduler utilizes this information to intelligently assign resources to users over parts of the bandwidth that maximize their signal-to-noise ratios (and spectral efficiency). In other words, the end-to-end performance of a multi-carrier system like LTE relies significantly on sub-carrier allocation techniques and transmission modes. LTE allows for different opportunistic scheduling techniques; a source of significant product differentiation between competing companies.
The multiple input multiple output (MIMO) is an advanced antenna technique to improve spectral efficiency and thereby boost overall system capacity. The MIMO technique uses a commonly known notation (M×N) to represent MIMO configuration in terms number of transmit (M) and receive antennas (N). The common MIMO configurations used for various technologies are: (2×1), (1×2), (2×2), (4×2), (8×2) and (2×4), (4×4), (8×4). In addition, 3GPP is discussing extending the number of antennas at the base station up to 16/32/64. The configurations represented by (2×1) and (1×2) are special cases of MIMO.
MIMO systems can increase the data carrying capacity of wireless systems. MIMO can be used for achieving diversity gain, spatial multiplexing gain, and beam-forming gain. For these reasons, MIMO is a part of the 3rd and 4th generation wireless systems. In addition, massive MIMO systems are under investigation for 5G systems.
FIG. 1 is a schematic diagram of multi-antenna transmission in LTE. More particularly, FIG. 1 illustrates data modulation 5A and 5B, antenna mapping 10, antenna ports 15, OFDM modulator 20, and antennas 25. Antenna mapping 10 can, in general, be described as a mapping from the output of data modulation 5A and 5B to different antenna ports 15. In the example illustrated in FIG. 1, there may be up to eight antenna ports 15. The input to antenna mapping 10 consists of modulation symbols (e.g., QPSK, 16QAM, 64QAM, 256QAM etc.) corresponding to the one or two transport blocks. More specifically, there is one transport block per Transport Time Interval (TTI), except for spatial multiplexing, in which case there may be two transport blocks per TTI. The output of the antenna mapping 10 is a set of symbols for each antenna port 15. The symbols of each antenna port 15 are subsequently applied to the OFDM modulator 20. In other words, the symbols of each antenna port 15 are mapped to the basic OFDM time-frequency grid corresponding to that antenna port 15. The output of OFDM modulators 20 may then be transmitted by antennas 25. For example, data may be transmitted by antennas 25 to a user equipment (UE).
3GPP LTE provides several different variations on MIMO techniques, from beamforming to spatial multiplexing or single antenna schemes through selection of one of ten Transmission Modes (TMs). These TMs are explained below.                Transmission Mode 1: Single Transmit Antenna Mode. This mode is mandatory for all terminals and used for eNode B which has only a single transmit antenna, for example small cell eNodeBs. This can also be used for macro eNodeBs in cases where using more than 1 Tx antenna is not feasible (e.g., certain antenna sharing scenarios with other 2G/3G technologies).        Transmission Mode 2: Open Loop Transmit Diversity Mode. In this mode, the same information is transmitted through multiple antennas, each with different coding/frequency resources. Alamouti codes are used with 2 antennas as the Space Frequency Block Codes (SFBC). This transmission scheme is also a common fallback mode to single layer transmission with dynamic rank adaptation in other transmission modes. This uses Space Frequency Block Coding (SFBC) for 2TX and SFBC+Frequency Shift Time Diversity (FSTD) STX for 4TX.        Transmission Mode 3: Open Loop Spatial Multiplexing with Cyclic Delay Diversity and Open Loop Transmit Diversity. This mode is also called open loop single user MIMO. As an open loop mode, this requires no PreCoding Matrix Indicator (PMI) but only rank is adapted. Due to its simplicity, this is the widely deployed mode during the initial deployments of 3GPP LTE.        Transmission Mode 4: Closed Loop Spatial Multiplexing (SU MIMO for rank 2 to 4). This has been the primary configuration for the majority of initial Release 8/9 deployments, operating while the channel has rank 2 to 4. It multiplexes up to four layers onto up to 4 antennas. To allow the UE to estimate the channels needed to decode multiple streams, the eNodeB transmits Common Reference Signals (CRS) on prescribed Resource Elements. The UE replies with the PreCoding Matrix Indicator (PMI) indicating which precoding is preferred from the pre-defined codebook. This is used for Single User, SU-MIMO. When the UE is scheduled, a precoding matrix is selected and the UE is informed explicitly or implicitly which precoding matrix was used for the actual physical downlink shared channel (PDSCH) transmission.        Transmission Mode 5: Closed-Loop Multi-User MIMO for rank 2 to 4. This mode is similar to TM4 but for the multi-user case (where multiple users are scheduled within the same resource block).        Transmission Mode 6: Closed Loop Rank 1 Precoding. This mode uses PMI feedback from the UE to select the preferred (one layer) codebook entry (precoding vector) from the pre-defined rank 1 codebook. Since only rank 1 is used, beam-forming gain is expected in this mode but no spatial multiplexing gain.        Transmission Mode 7: Single Layer Beam-forming. In this mode, both the data and the Demodulation Reference Signals (DMRS) are transmitted with the same UE-specific antenna precoder so that the UE does not distinguish the actual number of physical antennas used in the transmission and it does not know the actual precoding weights used as in the classical beam-forming approach (TM6). TM7 is mainly used with TD-LTE where the downlink channel state is well characterized by uplink measurements, due to reciprocity.        Transmission Mode 8: Dual layer beam-forming. This mode was introduced in Release 9. TM8 does classical beam forming with UE-specific DMRSs, like TM7 but for dual layers. This permits the eNode B to weight two separate layers at the antennas so that beam-forming can be combined with spatial multiplexing for one or more UEs. The two layers can be targeted to one or two UEs.        Transmission Mode 9: 8 layer MU-MIMO introduced in Release 10 as part of LTE-Advanced. TM9 implements 2, 4 or 8 reference signals for measurements (CSI-RS) as well as 1 to 8 UE-specific reference signals for demodulation (DMRS). Hence, it is a generalization of TM8 for up to 8 layer transmission and the introduction of CSI-RS enhances the CSI feedback. It is suitable for MU-MIMO with dynamic switching from SU-MIMO. It is applicable to either TDD or FDD systems and it is mandatory for terminals of Release 10 or later.        Transmission mode 10: An enhancement of TM9 where the resources used interference measurements has been further defined by the introduction of CSI-IM. TM10 is optional for terminals of Release 11 or later.        
FIG. 2 illustrates an example signal flow diagram for downlink data transfer in LTE. At step 205, UE 110 receives pilot or reference signals transmitted by network node 115, such as an eNodeB. From the pilot or reference signals, UE 110 computes channel estimates, and then computes the parameters needed for channel state information (CSI) reporting. The CSI report may include, for example, a channel quality indicator (CQI), a precoding matrix indicator (PMI), and rank information (RI).
At step 210, UE 110 sends the CSI report to network node 115 via a feedback channel, such as, for example, the physical uplink control channel (PUCCH) or the physical uplink shared channel (PUSCH). The PUCCH may be used for periodic CSI reporting, while the PUSCH may be used for aperiodic reporting. A scheduler associated with network node 115 uses this information in choosing the parameters for scheduling of UE 110. At step 215, network node 115 sends the scheduling parameters to UE 110 in the downlink control channel called physical downlink control channel (PDCCH). At step 220, actual data transfer takes place from network node 115 to UE 110. Data transfer between network node 115 and UE 110 may continue for any suitable period of time. In certain circumstances, however, it may become necessary for UE 110 to be handed over from network node 115 to another network node (i.e., a target network node). The handover (HO) procedure is described in more detail below.
Downlink reference signals are predefined signals occupying specific resource elements within the downlink time-frequency grid. The LTE specification includes several types of downlink reference signals that are transmitted in different ways and used for different purposes by the receiving terminal:                Cell-specific reference signals (CRS): These reference signals are transmitted in every downlink subframe and in every resource block in the frequency domain, thus covering the entire cell bandwidth. The cell-specific reference signals can be used by the terminal for channel estimation for coherent demodulation of any downlink physical channel except for the physical multicast channel (PMCH) and for PDSCH in the case of transmission modes 7, 8, or 9. The cell-specific reference signals can also be used by the terminal to acquire CSI. Finally, terminal measurements on cell-specific reference signals are used as the basis for cell-selection and handover decisions.        Demodulation reference signals (DM-RS): These reference signals (also sometimes referred to as UE-specific reference signals), are specifically intended to be used by terminals for channel estimation for PDSCH in the case of transmission modes 7, 8, 9 or 10. The label “UE-specific” relates to the fact that each demodulation reference signal is intended for channel estimation by a single terminal. That specific reference signal is then only transmitted within the resource blocks assigned for PDSCH transmission to that terminal.        CSI reference signals (CSI-RS): These reference signals are specifically intended to be used by terminals to acquire CSI in the case when demodulation reference signals are used for channel estimation. CSI-RS have a significantly lower time/frequency density, thus implying less overhead, compared to the cell-specific reference signals.In addition to these reference signals, there are other reference signals such as Multimedia Broadcast Single Frequency Network (MBSFN) and positioning reference signals used various purposes.        
In LTE, the uplink control channel carries information about HARQ-ACK information corresponding to the downlink data transmission, and channel state information. The channel state information typically consists of RI, CQI, and PMI. Either PUCCH or PUSCH can be used to carry this information. Note that the PUCCH reporting is periodic and the periodicity of the PUCCH is configured by the higher layers, while the PUSCH reporting is aperiodic.
In LTE, the downlink control channel (PDCCH) carries information about the scheduling grants. Typically this includes the number of MIMO layers scheduled, transport block sizes, modulation for each codeword, parameters related to HARQ, and sub band locations. Note with DM-RS, there is no need to inform the selected precoding matrix, thereby saving the number of bits in the downlink control channel.
FIG. 3 is a schematic diagram of an example passive antenna array system 300. Passive antenna array system 300 includes baseband processing unit 310, power amplifier 320, power combiner/divider and phase shifter 330, and one or more antennas 340. In passive antenna array system 300, the baseband signals from baseband processing unit 310 are boosted by power amplifier 320, which is connected to the antennas 340 by long feedback cables 350. The use of long feedback cables 350 may result in cable losses, potentially leading to decreased performance and increased energy consumption. Furthermore, installation of passive antenna systems may be more complex, and may require more equipment space.
FIG. 4 is a schematic diagram of an exemplary active-array-antenna system 400. Active-array-antenna system (AAS) 400 may include radio frequency (RF) components, such as power amplifiers and transceivers, integrated with an array of antenna elements. For example, AAS 400 may include baseband processing unit 410, radio transceiver array 420, and antennas 430. Baseband processing unit 410 may perform the processing functions of AAS 400. Radio transceiver array 420 may include any suitable number of transceivers. Transceivers of radio transceiver array 420 may contain transmit chains and receive chains. Transmit chains may contain typical components such as filters, mixers, power amplifiers (PAs), and any other suitable components. Receive chains may contain typical components such as filtering, low noise amplifiers (LNAs), and any other suitable components. In some cases, the number of transmitters may not be equal to the number of receivers. AAS 400 may include any suitable number of antenna elements 430 in any suitable arrangement. For example, a number of potential physical arrangements exist, which may include (but are not limited to) uniform linear, matrix and circular. Typically, cross polarized arrangements are deployed with an antenna element for each polarization. AAS 400 offers several benefits compared to deployments having passive antennas connected to transceivers through long feeder cables, such as passive antenna array 300 illustrated in FIG. 3. For example, by using active antenna array 400, cable losses may be reduced, leading to improved performance and reduced energy consumption. As another example, the installation may be simplified, and the required equipment space may be reduced.
AAS 400 may have numerous applications. As one example, AAS 400 may be able to perform one or more of cell specific beamforming, user specific beamforming, vertical sectorization, massive MIMO, and elevation beamforming. AAS 400 may also be an enabler for further-advanced antenna concepts, such as deploying large numbers of MIMO antenna elements at an eNodeB. For these reasons, 3GPP started a study item investigating the feasibility to increase the number of transmit antennas to 16/32/64 for various purposes, as well as extending the CSI feedback to support 2-dimensional antenna arrays where the up to 64 eNode B antenna ports are distributed both in vertical and horizontal directions. When the number of antennas is increased beyond a threshold (e.g., 64 or beyond), however, the overhead due to the minimum number of reference signals configured based on traditional approaches also becomes very high.